Antenna direction control system

ABSTRACT

A satellite communications system comprises signal tracking for a directional antenna by detecting any directional error of the antenna relative to a pilot station, or predetermined offset angle therefrom, to provide phase modulated error signals which are combined with frequency modulated pilot signals radiated by the pilot station in order to pass composite error and pilot signals through a single receive channel including a rotary joint coupler connecting a spinning section and a despun antenna section of a spin-stabilized satellite. Any directional error of the despun antenna about the spin axis of a spinning satellite results in sum and difference signals being produced in the receive feeds of the antenna wherein the magnitude of the difference signal is proportional to the angle of deviation of the antenna pointing direction and the pilot station. The difference signal is phase modulated in an over-coupled, doubletuned, directional filter and combined with the sum signal to produce a composite signal which exhibits amplitude modulation in synchronism with the frequency modulation of the sum signal. The amplitude modulation index is proportional to the difference signal and hence the directional error; and the phase of the amplitude modulation relative to the phase of the frequency modulation indicates the sign of the error. After passing through the rotary coupler, the envelope of the AM signal is detected and compared in a phase detector, with a reference signal derived from the FM of the composite signal. The output of the phase detector is the directional error signal which provides a tracking signal for controlling a despin motor to correct the error in pointing of the antenna.

"L t1 titted tates Waist I l 1 1 3,7523% Rosen 1 Sept. 4, 1973 i 1ANTENNA DIRECTION CQNTRUL SYSTEM phase modulated error signals which arecombined I75] Inventor: Harold Ruse Santa Monica with frequencymodulated pilot signals radiated by the Calm pilot station in order topass composite error and pilot signals through a single receive channelincluding a ro- 1 Asslgneel ng p y, Culver tary joint coupler connectinga spinning section and a clty, Callfdespun antenna section of aspin-stabilized satellite. [22] Filed. July 2 1970 Any directional errorof the despun antenna about the 21 Appl. No.: 51,372

Primary Examiner-Samuel Feinberg Assistant ExaminerRichard E. BergerAttorney-James K. Haskell and Richard 1. Rengel [57] ABSTRMCT Asatellite communications system comprises signal tracking for adirectional antenna by detecting any directional error of the antennarelative to a pilot station, or predetermined offset angle therefrom, toprovide spin axis of a spinning satellite results in sum and differencesignals being produced in the receive feeds of the antenna wherein themagnitude of the difference signal is proportional to the angle ofdeviation of the antenna pointing direction and the pilot station. Thedifference signal is phase modulated in an over-coupled, doubletuned,directional filter and combined with the sum signal to produce acomposite signal which exhibits amplitude modulation in synchronism withthe frequency modulation of the sum signal. The amplitude modulationindex is proportional to the difference signal and hence the directionalerror; and the phase of the amplitude modulation relative to the phaseof the frequency modulation indicates the sign of the error. Afterpassing through the rotary coupler, the envelope of the AM signal isdetected and compared in a phase detector, with a reference signalderived from the FM of the composite signal. The output of the phasedetector is the directional error signal which provides a trackingsignal for controlling a despin motor to correct the error in pointingof the antenna.

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m a 'nr 3 ANTENNA DERECTION CONTROL SYSTEM CROSS-REFERENCES TO RELATEDAPPLICATIONS The following copending applications have a commonassignee:

US. patent application Ser. No. 51,423, filed July I, 1970, of Harold A.Rosen and James S. Ajioka, entitled SHAPED BEAM ANTENNA, now Pat. No.3,680,143;

US. patent application Ser. No. 51,869, filed July 2, 1970, of Harold A.Rosen, entitled DUAL MODE RO- TARY MICROWAVE COUPLER, now Pat. No.3,668,567; and

US. patent application Ser. No. 657,393, filed July 31, 1967, of C. M.Mears, Jr. et al., now abandoned.

BACKGROUND OF THE INVENTION In domestic communication satellites foraugmenting internal communications of a country, high communicationcapability can be provided at substantial lower cost by simplificationsof the operation of numerous systems and subsystems. For example,reduction in weight of the satellite or communication payload permitsthe use of smaller boosters for insertion of the satellite into orbit aswell as improving the performance and reliability of the system.Improved reliability, in turn, is another benefit which reduces the needfor redundancy to further reduce cost and weight. Accordingly,advantages obtained by simplification of various subsystems are not indirect ratio to the cost reduction of the individual subsystem.

In order to obtain optimum communication coverage over a country beingserved, a beam pattern is provided by the communication antenna and theantenna is aimed to provide this coverage. Antenna direction controlbecomes increasingly complex as the aiming accuracy and desired lifetimeincreases. In one of the prior antenna control systems for aspin-stabilized satellite, for example, redundant earth sensors and sunsensors provide the basic sensing elements, and a digital computerderives the steering signals for a despin motor to control orientationof the antenna for efficient communication coverage of a designatedarea. In addition to the added weight of these sensors and computer inthe spacecraft, the prior art antenna direction control system requiresprecise clocking signals generated by a local clock for the sun sensor.Further, the earth sensors operating in the eclipse mode are subject tonoise and the combined sun and earth sensor systems permit antennadirection control errors due to thermal distortion of the antenna, forexample.

SUMMARY In order to overcome the difficulties of sun-earth sensorsystems and other prior art on-board systems, the present inventionprovides for placing a substantial portion of the complex systemcomponents and circuitry on the ground at a pilot station. In thespacecraft or satellite, orientation of a directive communicationantenna for an efficient communications system is simplified bydetecting the angle of arrival of a pilot signal radiated from the pilotstation and diplexing of the pilot signals and any directional errordetected, for transmission thereof along with other received signals, bya single channel provided between the antenna and receivers forcommunications and pilot signals. Thus, in spinstabilized satallites,both received and transmitted communications signals and pilot orcommand signals can be passed through a single rotary joint couplerbetween a despun antenna section and a spinning section of the satellitecarrying receivers and other equipment. As a result, the simplifiedcommunications system not only provides improved performance but alsoprovides greater reliability to reduce the possibility of prematurefailure of the satellites. Accordingly, an object of the invention isthe provision of an improved antenna direction control system having theforegoing features and advantages.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial presentation ofa spin-stabilized communications satellite in stationary orbit andhaving a despun directional antenna pointing toward a pilot stationto'augment internal communications in an area;

FIG. 2 is a schematic block diagram of a satellite communications systemincluding the improved antenna direction control system according to thepreferred embodiment of the invention;

FIG. 3a illustrates vectorially the derivation of typical amplitudemodulation of the pilot signal by phase modulation of the difierenceerror signal through frequency selective delay in synchronism withfrequency modulations present in the difierence error signal andcombining sum and difference signals; and

FIG. 3b shows typical waveforms of the combined sum and differencesignals exhibiting amplitude modulation of the sum signal illustratedpictorially in FIG. 3b and frequency modulation of the signals.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, aspin-stabilized 4 is shown in synchronous orbit and having a spin axisparallel to the spin axis of the earth. One or more antennas including adirectional antenna are mounted in a despun section of the satellite andcoupled to the main spinning section 5 by a rotary joint coupler 7. Asuitable despin motor mounted in the satellite 4 and mechanicallyconnected to the antenna 110 by the coupler 7 provides for despinning ofthe antenna section. Further, in accordance with the direction controlsystem described in detail hereinafter, despinning of the antennasection is controlled so that the directional antenna 10 is pointedtoward a pilot station 8 located in the area in which communications areaugmented by the satellite communications system including repeaters inthe satellite 4. The spinning section 5 of the satellite carriescommunication, telemetry, command, control, power and apogee motorsystems. Since the orbit of the satellite is predetermined, theelevation of the directional antenna 10 is preset although adjustmentthereof may be provided by suitable means controlled in response tocommand signals from the pilot station 8. Accordingly, the directioncontrol system provides for direction control about the spin axis ofsatellite 4 by controlling the despinning of the antenna section by thedespin motor.

Referring to FIG. 2, the block diagram shows the preferred satellitecommunications system for satellite 4 to comprise satellitecommunication equipment for receiving communications and pilot orcommand signals from ground stations 8 including the pilot station. Thissystem provides for control of the orientation of the despun directionalantenna 10 about the spin axis of the satellite for maximum efficiencyin transmission of communication signals, operating in dual mode, or bya separate antenna (not shown) for transmitting communication and othersignals.

The directional antenna includes feed horns 10a and 10b which are offsetfrom the axis ofa single parabolic reflector 100 to supply individualsignals to separate inputs of a hybrid junction 11. In response to afrequency modulated pilot signal radiated from the pilot ground station8, the feed horns 10a and 10b will supply signal inputs to the hybridjunction 11 which vary in relative magnitude (and phase) whenever thedirectional antenna 10 deviates from the direction of the pilot station8. During the launch and transfer orbit phase, commands are receivedfrom the pilot station by a toroidal beam antenna 9 (FIG. 1) located atthe very top of the satellite antenna section. During normal onstationoperation, commands are received by the highly directional antenna 10which also serves for receipt of communication signals from the area onearth being served by the satellite 4. In the receive mode, an antennafeed 12 including directional filter 13 provides for detecting anydirectional error of antenna 10 about the spin axis of the satellite.The toroidal beam antenna 9 and directional antenna 10 are, preferably,orthogonally polarized in order to reduce the interference between them,and the signals received by the two antennas 9 and 10 are summed by useof a directional filter (not shown), along with communications signalsreceived, and fed through a single coaxial channel provided by a coaxialcable 14 passing axially through the rotary joint coupler 7. On thespinning section side of the rotary joint coupler 7, the pilot signal isseparated from the communications signal by a directional filter 15 andintroduced into hybrid junction 16 for splitting the power forsimultaneously driving pilot receiver 18 and redundant pilot receiver20.

The pilot receivers 18 and 20 each include an input mixer 21, coupled toa local oscillator 22, which downconverts the pilot signal of microwavefrequency to IF for amplification by an IF amplifier 23. The pilotsignal, for example, comprises a 6 GHz carrier, frequency modulatedbetween peak-to-peak frequency deviations of 4 MHz by a 10 KHz audiotone which is demodulated by an FM discriminator 24 coupled to theoutput of the IF amplifier 23. The demodulated audio or pilot tonesprovided at the output of FM discriminator 24 are coupled to commanddecoders 25. The command decoders 25 may include tone filters, thresholddetectors and a digital register having suitable logic for addressingand executing the required commands.

Considering now details of the directional control system aspects of thesystem shown in FIG. 2, the frequency modulated pilot signal radiatedfrom the ground pilot station 8 and received by the directional antenna10 supplies signals of equal magnitude to inputs of hybrid junction 11whenever the antenna 10 is pointed directly at the pilot station 8.Deviation of the antenna 10 to the right or left of the pilot stationresults in signals of different magnitude at receive ports of feed horns10a and 10b and a difference signal A appearing at the difference signaloutput 11b of the hybrid junction 11. This signal appearing in thedifference arm of the hybrid junction is proportional to the magnitudeof the angle-error between the pointing direction of antenna 10 aboutthe spin axis of satellite 4 and the direction of the pilot station. Theother output 1 la of hybrid junction 11 is the sum arm. The signalsreceived from the pilot station 8 by the feed horns 10a and 10b areadded in the sum arm to provide a sum signal 2. The difference signal Ais coupled to the directional filter 13, which is over-coupled anddouble-tuned to provide periodic phase reversals of the differencesignal A at the frequency modulation rate at the output thereof. Thephase reversal bandwidth of the directional filter 13 is equal to thepeak-to-peak deviation of the frequency modulated pilot signal radiatedfrom the pilot station 8. For example, the peak-to-peak deviation of thepilot signal is 4 MHz and the peak-to-peak phase shift bandwidth of thedirectional filter is also 4 MHz.

As a result of the double tuning of the directional filter 13, thefilter produces a frequency selective delay providing a 180 phase shiftacross the 4 MHZ bandwidth. The directional filter 13 comprises a twocavity resonator capable of providing this 180 phase shift over the 4MHz peak-to-peak frequency deviations. Accordingly, the higher peakfrequency f; of the difference signal A will be delayed to reverse thephase thereof (180 phase shift) relative to the lower peak frequencydeviation f Any constant delay will be com pensated for by acorresponding delay of the sum signal 2 and the difference in delaybetween peaks f and f provides a 180 relative phase shift between f andf As the frequency modulation of the pilot signal varies about the pilotcarrier f between peak frequency deviations f, and f the differencesignal A is phase modulated in passing through the coupling paths offilter 13 to undergo periodic phase reversals at the frequencymodulation rate. When this phase modulated signal output of the filter13 is combined with the sum signal 2 in the sum transmission line, thecombined or resultant signal exhibits an amplitude modulation insynchronism with the frequency modulation of the pilot signals. Theincidental phase modulation is ignored. The combined signal is then fedto the pilot receivers 18 and 20 through the coaxial cable 14' of therotary joint coupler 7. For a more detailed description of directionalfilters of the type shown herein, reference is made to the text entitledMicrowave Filters, Impedance-Matching Networks, and Coupling Structures"by G. L. Matthaei et al., published by McGraw-I-Iill, Inc. and dated1964 and more particularly Section 14.02 et seq. entitled WaveguideDirectional Filters".

Referring now to FIG. 2 and diagrams in FIGS. 3a and 3b for adescription of the operation of the improved antenna direction controlsystem of the present invention, in pilot signal tracking, the FM pilotsignal received from the pilot station 8 by the antenna feed horns 10aand 10b is supplied from receive ports thereof to respective inputs ofthe hybrid junction 11 wherein the inputs are added in the sum arm ofthe junction to provide the sum 2 thereof at output 11a. Any differencebetween signal outputs at these receive ports in the pilot FM band ispassed through a difference arm of the hybrid junction 11 and suppliedfrom output arm 11b to the input of the directional filter l3 and amatched impedance load 13a in parallel therewith. When the antenna 10 ispointed directly at the pilot station 8, there is zero angle-error andthe pilot signals supplied from respective feed horns 10a and 10b areequal. However, an angle-error 4R (pilot station to the right of thepointing direction of antenna 10) results in a difference signal A of afirst phase R;

and an angle-error 4L (pilot station to the left of the pointingdirection of antenna results in a reversal in the polarity of thedifference signal A to a second phase 41L, i.e. shifted in phase by 180.

As shown in FIG. 3b, the peak-to-peak deviation f,-f of the FM pilotsignal corresponds to the 180 peak-to-peak, phase shift bandwidth of thedoubletuned, directional filter 13. The difference signal A, passed bythe filter 13 therefore undergoes periodic phase reversals at thefrequency modulation rate, i.e. phase modulated. When the phasemodulated difference frequency A, at the output of filter H3, iscombined with the sum signal 2, the resultant or combined signalsexhibit an amplitude modulation in synchronism with the frequencymodulation, wherein the amplitude modulation index so produced isproportional to the amplitude of the difference signal A and hence, themagnitude of the directional error. The phase of amplitude modulationsof the resultant signal, relative to the phase of the frequencymodulation, is determined by the polarity R, L of the difference signalAas indicated by the modulation envelopes of the waveform of theresultant signals in FIG. 3b.

To illustrate the foregoing, a directional angle-error L R is firstassumed wherein the antenna 10 is pointing to the right of the pilotstation 8 which is radiating an FM pilot signal, modulated with a highdeviation (e.g. 4 Ml-Iz) at an audio rate (e.g. 10 KHz). Antenna horns10a and 10b, being offset from the center line of the parabolicreflector 10c, supply output signals of different magnitudes torespective inputs of hybrid junction 11 which adds these antenna outputsignals in its sum arm to provide a sum output signal 2', at output Ila; and provide a difference signal A of the first phase R at output11b. As illustrated in FIG. 3a, the lower frequency deviation f of thedifference signal A (R) is passed by directional filter 13 without delayand combined in phase with the sum signal 2, adding to the sum signal 2,to produce the larger resultant R As the frequency varies sinusoidallyfrom f to f at the 10 KHz frequency modulation rate, as shown in FIG.3b, the difference signal A appearing at the output of the directionalfilter 13 undergoes a phase reversal at peak frequency f As shown by thevector diagraom in FIG. 3a for the higher frequency deviation f andangle-error LR, the difference signal A at the peak frequency fsubtracts from the sum signal to produce a resultant signal R During theperiod of a complete cycle of frequency modulation of the pilot signal,the combined signal exhibits amplitude modulation in synchronism withthe frequency modulation as shown in FIG. 3b (pilot station right). Theamplitude modulation index is proportional to the magnitude of thedifference signal A (and the directional angle-error LR) and the phaseof the AM, relative to frequency modulation, indicates that theangle-error is to the right of the pilot station 8.

The operation of the foregoing directional feed for an angle-error 4- Lin which the antenna 10 is pointing to the left of the pilot stationshould be evident from the foregoing description for the angle-error ZR. As indicated by the vector diagram of FIG. 3a, the difference signalA for angle-error 1 L is reversed in phase (L) whereby the differencesignal A of lower frequency peak deviation f is 180 out of phase withrespect to the frequency modulation of the sum signal and subtracts fromthe sum signal 2 to produce a smaller resultant signal R for peakdeviation f,. For delayed higher frequency deviationf (L), thedifference frequency A (L) at the peak deviation f is shifted in phase180 to be in-phase with the frequency modulations of the sum signal 2 toadd thereto and produce a larger resultant signal R The combined sum 2and difference signals A (dJL) for angle-error 4L exhibit amplitudemodulations reversed in phase for pilot station to the left of thepointing direction of antenna 10, as evident from a comparison of theenvelopes of the signals in FIG. 3b.

In the pilot receiver 18, the combined signal passed thereto is coupledto an AM detector 26 after passing through mixer 21 and amplifier 23.The envelope of the amplitude modulated IF signal is detected by the AMdetector 26 and compared in a phase detector 28 with a reference signalsupplied from the frequency discriminator 24. The output of the phasedetector 28 is proportional in amplitude and related in sign to thedirectional error of the antenna H0. The signal at the output of thephase detector 28, including its rate, provides the tracking inputs to adirectional signal amplifier 29, preferably including a motor poweramplifier, whose output is connected to despin motor 30. A despin motorof the type referred to herein, which is responsive to the error signalprovided at the output of amplifier 29, is shown and described in theaforementioned copending application Ser. No. 657,393.

The preferred phase detector 28 is of the linear phase detector typewhich includes a lower pass filter for passing the low frequency or DCcomponent to provide an output voltage proportional to the cosine of thephase difference of the reference and error signal. Thus, the preferredphase detector 28 is distinguished from other phase detectors whichproduce a voltage proportional to the phase difference. The presentphase detector 28 differs from flip-flop phase detectors or sample holdphase detectors in that these latter conventional phase detectorsproduce a voltage proportional to the phase difference whereas thepresent linear phase detector utilizes multiplier approach which willproduce a voltage proportional to the cosine of the phase difference ofreference and detected AM signals.

In general, when operating in the receive mode, the antenna feed 12including the hybrid junction ll passes the communication signals alongwith the combined AM-FM sum signals through a coaxial cable M whichpasses through the center of the rotary joint coupler 7. Thecommunication signals are separated from the pilot signal carrier in adirectional filter having an output coupled to the communicationsreceivers 32 through a bandpass filter 34. Transmitters 36 are coupledto antenna 10, or a separate transmitting antenna having the samedirection as antenna Ml, through the rotary joint coupler 7 by outerwaveguides which are described in detail in my aforementioned copendingapplication entitled DUAL MODE ROTARY MICRO- WAVE COUPLER being filedconcurrently herewith. In the coaxial cable 14, 4 GI-Iz chokes on anouter conductor thereof minimize interaction with the transmit signalspassed through a surrounding waveguide of the coupler 7.

In the control system for providing directional control of the antenna10 in order to maintain pointing thereof at the pilot station 8, thepreferred directional signal amplifier 29, as noted earlier, includes amotor power amplifier. In further discussion thereof, the despin motor30 includes a motor bearing assembly which contains a brushless DCmotor, known as a resolver commutated motor, two sets of angular contactbearings and a master index pulse generator. A motor of this type ismanufactured by Aeroflex Corporation, Long Island, N.Y., and isidentified by number TFR 47- The motor power amplifier includes acarrier oscillator and demodulators. The input error signals from thephase detector 28 modulate the carrier which is then resolved in sineand cosine components by a resolver associated with the motor windings.The resolved signals are synchronously demodulated and low frequencyoutputs are applied to the motor 30 to produce the desired torque. Inorder to provide an offset in the antenna direction for aiming antennaat locations other than the pilot signal direction, by ground command,an offset pointing capability can be provided by a storage register incombination with a digital-toanalog converter having an output which issummed with the error signal at the input to the motor power amplifier.This offset provided by pointing command can also be used during initialacquisition of communications as the satellite comes into view of thepilot station.

While a preferred embodiment of the invention has been disclosed, itshould be clear that the present invention is not limited thereto asmany variations will be readily apparent to those skilled in the art.Thus, although it is readily apparent, the preferred embodiment of theinvention as described and disclosed is particularly suitable for asatellite communication system; it is also readily apparent that thesystem of the present invention is usable on the surface of the earth asa signal tracking station having a directional control system for anantenna which provides for detecting a directional error of the antennarelative to remote fixed pilot ground stations or vehicles moving on orimmediately above the earth's surface and providing a correspondingradiated signal FM and it is desired to maintain the antenna directed toany one of the ground stations or moving vehicle source of signals.Alternatively, the antenna direction control system of the presentinvention can be disposed on the moving vehicle or on immediately abovethe earths surface for directing the vehicles antenna toward any groundstation providing the source of FM signals. Accordingly, the system ofthe present invention provides for multiplexing plurality of frequencymodulated signals to provide a composite signal exhibiting amplitudemodulation as a function of at least one of the frequency modulatedsignals having a frequency modulation rate for use in signal tracking orother multiplexing systems in communications systems.

What is claimed is:

1. In a communication system, the combination comprising:

a directional antenna for receiving a radiated frequency modulatedsignal from a signal source, said signal having a period forpeak-to-peak frequency deviations, said antenna having antenna feedsresponsive to the radiated signal received to produce individual signaloutputs including individual signal outputs of different magnitudewhenever the pointing direction of the antenna deviates from thedirection of said radiated signal;

means coupled to said antenna feeds, said means including a sum armresponsive to individual signal outputs to add said signal outputs toproduce a sum signal and a difference arm responsive to individualsignal outputs of different magnitudes to produce a difference signalhaving a magnitude corresponding to the degree of deviation of thepointing direction of said antenna from the direction of said source;

frequency selective delay means having a band-width correspondingapproximately to said peak-to-peak frequency deviations of said radiatedsignal, said delay means being responsive to said difference signal toproduce a phase modulated signal having a magnitude corresponding tosaid degree of deviation of said antenna and periodic phase reversalswherein the period of said phase reversals corresponds to the period forpeak-to-peak frequency deviation of the radiated signal; and

means for combining said sum signal and phase modulated signal toprovide a composite signal exhibiting amplitude modulation includingsaid periodic phase reversals of said phase modulated signals.

2. The combination according to claim 1 which further includes means fordetecting the amplitude of said composite signal to provide an antennadirectional error signal corresponding in magnitude to the degree ofdeviation of the pointing direction of the antenna from the direction ofsaid radiated signal.

3. The combination according to claim 1 in which said means coupled tosaid antenna feeds includes means responsive to a change in pointingdirection of the antenna which crosses over the direction of saidradiated signal to produce a reversal in polarity of said differencesignal and a corresponding reversal in polarity of said amplitudemodulation of said composite signal.

4. The combination according to claim 3 which further includes meansresponsive to said composite signal for producing an error signal havinga magnitude proportional to said degree of directional deviation of saidantenna and polarity corresponding to the pointing direction of theantenna relative to the direction of the radiated signal.

5. The combination according to claim 4 which further comprises aspin-stabilized vehicle having a predetermined spin axis transverse tothe direction of a source of said radiated signal including a spinningsection, a despun section, and means including a rotary joint couplerand a despin motor for controlling the pointing of said directionalantenna disposed on said despun section about said spin axis, saiddespin motor being responsive to said error signal to maintain thepointing direction of the antenna about the spin axis toward the sourceof said radiated signal.

6. The combination according to claim 5 in which said directionalantenna comprises antenna feed means including two feed horns and asingle parabolic reflector, and said feed horns are offset from the axisof said reflector to provide receive signal outputs of differentmagnitudes whenever the pointing direction of the antenna is differentfrom the direction of said source of radiated signals and the magnitudeof the difference of said receive signal outputs is proportional to theangle between the pointing direction and the direction of the source ofradiated signals.

7. The combination according to claim 6 in which said feed horns aredisposed to produce difference signals of opposite polaritycorresponding to the antenna pointing directions on either side of saidsource of radiated signal.

8. A system for multiplexing a plurality of frequency modulated signalsto provide a composite signal exhibiting amplitude modulation as afunction of magnitude of at least one of the frequency modulated signalshaving a frequency modulation rate comprising:

frequency selective delay means capable of producing phase reversalsbetween peak frequency deviations of frequency modulation and responsiveto at least one of said plurality of frequency modulated signals capableof varying in magnitude, to phase modulate said latter signal includingperiodic phase reversals at the frequency modulated rate;

combining means responsive to said phase modulated signal and another ofsaid plurality of frequency modulated signals to produce a compositesignal exhibiting amplitude modulation wherein the index of amplitudemodulation is a function of the magnitude of said frequency modulatedsignal capable of varying in magnitude; and one of said plurality offrequency modulated signals and said another of said plurality offrequency modulated signals have the same modulation rate to provideperiodic phase reversals of the amplitude modulation of said compositesignal. 9. A system for multiplexing a plurality of frequency modulatedsignals to provide a composite signal exhibiting amplitude modulation asa function of magnitude of at least one of the frequency modulatedsignals having a frequency modulation rate comprising:

frequency selective delay means capable of producing phase reversalsbetween peak frequency deviations of frequency modulation and responsiveto at least one of said plurality of frequency modulated signals capableof varying in magnitude, to phase modulate said latter signal includingperiodic phase reversals at the frequency modulated rate;

combining means responsive to said phase modulated signal and another ofsaid plurality of frequency modulated signals to produce a compositesignal exhibiting amplitude modulation wherein the index of amplitudemodulation is a function of the magnitude of said frequency modulatedsignal capable of varying in magnitude; and

said plurality of frequency modulated signals are signals of microwavefrequency and said frequency selective delay means comprises a microwavedirectional filter, double-tuned to said peak frequency deviations.

it). A system for multiplexing a plurality of frequency modulatedsignals to provide a composite signal exhibiting amplitude modulation asa function of magnitude of at least one of the frequency modulatedsignals having a frequency modulation rate comprising:

frequency selective delay means capable of producing phase reversalsbetween peak frequency deviations of frequency modulation and responsiveto at least one of said plurality of frequency modulated signals capableof varying in magnitude, to phase modulate said latter signal includingperiodic phase reversals at the frequency modulated rate; combiningmeans responsive to said phase modulated signal and another of saidplurality of frequency modulated signals to produce a composite signalexhibiting amplitude modulation wherein the index of amplimde modulationis a function of the magnitude of said frequency modulated signalcapable of varying in magnitude; and

said one of said frequency modulated signals comprises a differencesignal and said another of said plurality of frequency modulated signalscomprises a sum signal, and said system further includes means forderiving sum and difference signals from individual frequency modulatedsignals having the same frequency modulation rate.

11. The system according to claim 10 which further includes directionalantenna means including feed means responsive to a radiated frequencymodulated signal for producing separate frequency modulated signaloutputs including signal outputs of different magnitude whenever thepointing direction of said directional antenna means is different fromthe direction of said radiated signal.

12. The system according to claim in which the source of said radiatedsignal is the desired pointing direction of said antenna means and saidmeans for deriving sum and difference signals is responsive todeviations in the direction of the antenna to opposite sides of saidsource to produce a phase reversal of said difference signal andamplitude modulation of said composite signal.

13. A control system for aiming a directional antenna, disposed on adespun section of a spin-stabilized vehicle, toward a pilot stationsource of radiated frequency modulated signals comprising:

a directional antenna including feed means responsive to an angle ofdirectional error of said antenna relative to the direction of saidpilot station to produce individual signals of different magnitudewherein the difference in magnitude corresponds to the magnitude of theerror angle;

means responsive to said individual signals for producing sum anddifference signals including a difference signal having a magnitudeproportional to the error angle and polarity corresponding to thedirection of aiming of the antenna relative to the pilot station;

means for phase modulating said difference signal to produce periodicphase reversals of said difference signal at the frequency modulationrate of said pilot signals;

combining means for combining the sum and phase modulated differencesignals to produce a composite signal exhibiting amplitude modulation insynchornism with the frequency modulation of the pilot signal whereinthe index of amplitude modulation is proportional to the magnitude ofthe difference signal and the phase of the composite signal isdetermined by the polarity of the difference signal;

rotary coupling means including a coaxial cable for coupling saidcomposite signal from the combining means disposed on the despun sectionto the spinning section of said vehicle,

means disposed on said despun section and coupled to said rotarycoupling means for detecting the index of amplitude modulation and phaseof said amplitude modulation relative to said frequency modulation toproduce an error signal having a magnitude proportion to the errorangle; and

means including a despin motor responsive to said error signal tocontrol the despinning of said despun section to maintain said antennaaimed toward said pilot station.

1. In a communication system, the combination comprising: a directionalantenna for receiving a radiated frequency modulated signal from asignal source, said signal having a period for peak-to-peak frequencydeviations, said antenna having antenna feeds responsive to the radiatedsignal received to produce individual signal outputs includingindividual signal outputs of different magnitude whenever the pointingdirection of the antenna deviates from the direction of said radiatedsignal; means coupled to said antenna feeds, said means including a sumarm responsive to individual signal outputs to add said signal outputsto produce a sum signal and a difference arm responsive to individualsignal outputs of different magnitudes to produce a difference signalhaving a magnitude corresponding to the degree of deviation of thepointing direction of said antenna from the direction of said source;frequency selective delay means having a band-width correspondingapproximately to said peak-to-peak frequency deviations of said radiatedsignal, said delay means being responsive to said difference signal toproduce a phase modulated signal having a magnitude corresponding tosaid degree of deviation of said antenna and periodic phase reversalswherein the period of Said phase reversals corresponds to the period forpeak-to-peak frequency deviation of the radiated signal; and means forcombining said sum signal and phase modulated signal to provide acomposite signal exhibiting amplitude modulation including said periodicphase reversals of said phase modulated signals.
 2. The combinationaccording to claim 1 which further includes means for detecting theamplitude of said composite signal to provide an antenna directionalerror signal corresponding in magnitude to the degree of deviation ofthe pointing direction of the antenna from the direction of saidradiated signal.
 3. The combination according to claim 1 in which saidmeans coupled to said antenna feeds includes means responsive to achange in pointing direction of the antenna which crosses over thedirection of said radiated signal to produce a reversal in polarity ofsaid difference signal and a corresponding reversal in polarity of saidamplitude modulation of said composite signal.
 4. The combinationaccording to claim 3 which further includes means responsive to saidcomposite signal for producing an error signal having a magnitudeproportional to said degree of directional deviation of said antenna andpolarity corresponding to the pointing direction of the antenna relativeto the direction of the radiated signal.
 5. The combination according toclaim 4 which further comprises a spin-stabilized vehicle having apredetermined spin axis transverse to the direction of a source of saidradiated signal including a spinning section, a despun section, andmeans including a rotary joint coupler and a despin motor forcontrolling the pointing of said directional antenna disposed on saiddespun section about said spin axis, said despin motor being responsiveto said error signal to maintain the pointing direction of the antennaabout the spin axis toward the source of said radiated signal.
 6. Thecombination according to claim 5 in which said directional antennacomprises antenna feed means including two feed horns and a singleparabolic reflector, and said feed horns are offset from the axis ofsaid reflector to provide receive signal outputs of different magnitudeswhenever the pointing direction of the antenna is different from thedirection of said source of radiated signals and the magnitude of thedifference of said receive signal outputs is proportional to the anglebetween the pointing direction and the direction of the source ofradiated signals.
 7. The combination according to claim 6 in which saidfeed horns are disposed to produce difference signals of oppositepolarity corresponding to the antenna pointing directions on either sideof said source of radiated signal.
 8. A system for multiplexing aplurality of frequency modulated signals to provide a composite signalexhibiting amplitude modulation as a function of magnitude of at leastone of the frequency modulated signals having a frequency modulationrate comprising: frequency selective delay means capable of producingphase reversals between peak frequency deviations of frequencymodulation and responsive to at least one of said plurality of frequencymodulated signals capable of varying in magnitude, to phase modulatesaid latter signal including periodic phase reversals at the frequencymodulated rate; combining means responsive to said phase modulatedsignal and another of said plurality of frequency modulated signals toproduce a composite signal exhibiting amplitude modulation wherein theindex of amplitude modulation is a function of the magnitude of saidfrequency modulated signal capable of varying in magnitude; and one ofsaid plurality of frequency modulated signals and said another of saidplurality of frequency modulated signals have the same modulation rateto provide periodic phase reversals of the amplitude modulation of saidcomposite signal.
 9. A system for multiplexing a plurality of frequencymodulated signals to provide a composite signal eXhibiting amplitudemodulation as a function of magnitude of at least one of the frequencymodulated signals having a frequency modulation rate comprising:frequency selective delay means capable of producing phase reversalsbetween peak frequency deviations of frequency modulation and responsiveto at least one of said plurality of frequency modulated signals capableof varying in magnitude, to phase modulate said latter signal includingperiodic phase reversals at the frequency modulated rate; combiningmeans responsive to said phase modulated signal and another of saidplurality of frequency modulated signals to produce a composite signalexhibiting amplitude modulation wherein the index of amplitudemodulation is a function of the magnitude of said frequency modulatedsignal capable of varying in magnitude; and said plurality of frequencymodulated signals are signals of microwave frequency and said frequencyselective delay means comprises a microwave directional filter,double-tuned to said peak frequency deviations.
 10. A system formultiplexing a plurality of frequency modulated signals to provide acomposite signal exhibiting amplitude modulation as a function ofmagnitude of at least one of the frequency modulated signals having afrequency modulation rate comprising: frequency selective delay meanscapable of producing phase reversals between peak frequency deviationsof frequency modulation and responsive to at least one of said pluralityof frequency modulated signals capable of varying in magnitude, to phasemodulate said latter signal including periodic phase reversals at thefrequency modulated rate; combining means responsive to said phasemodulated signal and another of said plurality of frequency modulatedsignals to produce a composite signal exhibiting amplitude modulationwherein the index of amplitude modulation is a function of the magnitudeof said frequency modulated signal capable of varying in magnitude; andsaid one of said frequency modulated signals comprises a differencesignal and said another of said plurality of frequency modulated signalscomprises a sum signal, and said system further includes means forderiving sum and difference signals from individual frequency modulatedsignals having the same frequency modulation rate.
 11. The systemaccording to claim 10 which further includes directional antenna meansincluding feed means responsive to a radiated frequency modulated signalfor producing separate frequency modulated signal outputs includingsignal outputs of different magnitude whenever the pointing direction ofsaid directional antenna means is different from the direction of saidradiated signal.
 12. The system according to claim 10 in which thesource of said radiated signal is the desired pointing direction of saidantenna means and said means for deriving sum and difference signals isresponsive to deviations in the direction of the antenna to oppositesides of said source to produce a phase reversal of said differencesignal and amplitude modulation of said composite signal.
 13. A controlsystem for aiming a directional antenna, disposed on a despun section ofa spin-stabilized vehicle, toward a pilot station source of radiatedfrequency modulated signals comprising: a directional antenna includingfeed means responsive to an angle of directional error of said antennarelative to the direction of said pilot station to produce individualsignals of different magnitude wherein the difference in magnitudecorresponds to the magnitude of the error angle; means responsive tosaid individual signals for producing sum and difference signalsincluding a difference signal having a magnitude proportional to theerror angle and polarity corresponding to the direction of aiming of theantenna relative to the pilot station; means for phase modulating saiddifference signal to produce periodic phase reversals of said differencesignal at the frequency modulation rate of said piLot signals; combiningmeans for combining the sum and phase modulated difference signals toproduce a composite signal exhibiting amplitude modulation insynchornism with the frequency modulation of the pilot signal whereinthe index of amplitude modulation is proportional to the magnitude ofthe difference signal and the phase of the composite signal isdetermined by the polarity of the difference signal; rotary couplingmeans including a coaxial cable for coupling said composite signal fromthe combining means disposed on the despun section to the spinningsection of said vehicle, means disposed on said despun section andcoupled to said rotary coupling means for detecting the index ofamplitude modulation and phase of said amplitude modulation relative tosaid frequency modulation to produce an error signal having a magnitudeproportion to the error angle; and means including a despin motorresponsive to said error signal to control the despinning of said despunsection to maintain said antenna aimed toward said pilot station.